Sains Malaysiana 40(6)(2011): 577–585

Pull-Out and Tensile Strength Properties of Two Selected Tropical Trees(Ciri-Ciri Tarikan-Keluar dan Kekuatan Tegangan Dua Pokok Tropika Terpilih)

NORMANIZA OSMAN*, MOHAMAD NORDIN ABDULLAH & CHE HASSANDI ABDULLAH

ABSTRACT

A bioengineering technique is gradually being used as an alternative for slope stabilisation design. The effect of vegetation on soil strength, particularly in terms of root reinforcement aspects has therefore become a major interest. However, there is a lack of documentation on the root mechanical properties available especially in Malaysia. In this study, both pull-out and tensile strength of two tropical trees namely Leucaena leucocephala and Acacia mangium were investigated on different stem sizes. L. leucocephala performs the higher pullout strength than A. mangium. The results also show that pullout resistance is much affected by the root than the shoot profiles. In terms of tensile strength, the tensile strength decreases with increasing root diameter, implying the finer root diameter contribute to the higher tensile strength. In both parameters, L. leucocephala exhibits the highest value. The study suggests that L. leucocephala has an added value as a good potential slope plant for slope stabilization work as it exhibits outstanding root mechanical properties. Interestingly, the results also showed that the pullout force was much affected by the tensile strength. It can be concluded that some root and shoot properties do have a great impact on root mechanical properties such as tensile and pullout strengths.

Keywords: Lateral roots; pull-out; root morphologies; shoot morphologies; tap roots

ABSTRAK

Penggunaan kaedah bio-kejuruteraan semakin mendapat tempat sebagai alternatif dalam merekabentuk penstabilan cerun. Kesan vegetasi terhadap kekuatan tanah, terutamanya dalam aspek pengukuhan akar telah menjadi tumpuan utama. Walau bagaimanapun, terdapat kekurangan dalam mendokumentasikan ciri-ciri mekanikal akar terutamanya di Malaysia. Di dalam kajian ini, kedua-dua tarikan-keluar dan kekuatan tegangan bagi dua pokok tropika iaitu Leucaena leucocephala dan Acacia mangium dikaji pada saiz batang yang berbeza. L. leucocephala menunjukkan kekuatan tarikan-keluar yang lebih tinggi daripada A. mangium. Keputusan juga menunjukkan bahawa rintangan tarikan-keluar lebih dipengaruhi profil akar berbanding pucuk. Bagi kekuatan tegangan pula, kekuatan tegangan berkurang dengan peningkatan diameter akar, mengandaikan yang diameter akar yang halus menyumbang kepada kekuatan tegangan yang lebih. Bagi kedua-dua parameter, L. leucocephala mempunyai nilai yang paling tinggi. Kajian ini mencadangkan L. leucocephala mempunyai nilai tambah sebagai tumbuhan yang berpotensi untuk menstabilkan cerun di mana ia menunjukkan ciri-ciri akar yang menakjubkan. Menariknya, keputusan juga menunjukkan bahawa daya tarikan-keluar turut dipengaruhi oleh kekuatan tegangan. Dapat disimpulkan bahawa beberapa ciri akar dan pucuk mempunyai impak yang besar terhadap ciri mekanikal akar seperti tegangan dan kekuatan tarikan-keluar.

Kata kunci: Akar-akar lateral; akar-akar tunjang; morfologi akar; morfologi pucuk; tarikan-keluar

INTRODUCTION

Slopes in natural condition are prone to erosion as they are exposed to the erosion agents such as water, wind and others. The consequences of big scale erosion are unpredictable and can cause fatality. In most cases, slope stability is affected by the steepness or slope gradient, soil texture and bedrock underneath and also the rate of rainfall. Areas with high annual rainfall such as tropical humid climate basically face problems in slope stability. High intensity of rainfall leads to saturation of the soil and the saturated slope become unstable as its factor of safety is usually less than 1.0 (Van Beek et al. 2005). Among the factors considered as resistant forces against big scale erosion and landslide on slope are root tensile strength, root shear strength and pull-out strength

whereas the compound strength generated by both soil and root (Abe & Iwamoto 1986). The conventional methods in managing and restoring the slope stability such as soil nailing, key stones, masonry wall, netting, and rubble pitching are often used to solve the problems. However, the use of vegetation in restoring the slope stability had gained much interests as many researchers recognize the long term potential and advantages of vegetation in maintaining the slope stability (Mitsch 1998). Moreover, it has been accepted as a low cost solution especially in humid tropical areas (Collison et al. 1995). Hence, the role of root strength is important in stabilizing steep hill slopes. The slopes are generally stable if they are covered by shrubs whose roots anchor into the

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soil. It appears that root systems mechanically reinforce soil by transferring shear stress in the soil to tensile resistance in the roots. In addition, the number and size of roots which cross the slip surface are extremely important (Cammerat et al. 2005; Reubens et al. 2007; Van Beek et al. 2005) and the root architecture, especially the branching pattern has a close relationship with the strength of anchorage (Stokes et al. 1996). Apart from that, the mechanical characteristic of roots is that they are strong in tension and soils. On the other hand, they are strong in compression and weak in tension (De Baets et al. 2008). This combined effect of soil and roots, producing a composite material in which the roots are fibres of relatively high tensile strength and adhesion embedded in a matrix of lower tensile strength soil mass, resulting in a reinforced soil. Therefore, it is the tensile of the roots which contribute to the overall strength of the soil-root composite. Despite several studies and recommendations on the effects of roots, the bioengineering application of deep-rooted shrubs and trees has not been sufficiently carried out in Malaysia. Therefore, this research was aimed to study the engineering properties of the root of Malaysian species. Knowing the influence of plant root pullout and tensile strengths towards the reinforcing effect, a series of pull-out and tensile tests have been conducted on the roots of selected trees namely; L. leucocephala and A. mangium. The relationship between the engineering, the shoot and the root properties was also investigated.

MATERIALS AND METHODS

The assessment of the root strength of the species was conducted by means of field and laboratory tests. The plants for both pull-out and tensile test experiments were obtained from a flat land approximately 1350 m2 located at the Institute of Mathematical Sciences, University of Malaya (3° 07’ 51” N and 101°39’25.9” E). The site was observed to have abundance of the plants studied. Based on the grain size distribution curve, the soil is described as silty sand and its physical properties are tabulated in Table 1. Details on the plant species studied are tabulated (Table 2).

TEST METHODS

Pullout Test The location of the selected plant sample was identified and marked using the stem tagging. Nine samples were chosen for each species, three for each stem diameter treatment (0-20 mm, 20-40 mm and 40-60 mm). Before setting up the equipment, the ground surface around the plant had been cleared. After measuring height of the plant, the stem of the plant was cut approximately at 10 cm above the root crown, to ensure that the barrel could be placed properly and the wedges were able to fully grip the plant stem as tight as possible. Appropriate wedges were chosen depending on the size of the stem and root crown diameter (Figure 1).

TABLE 1. Soil physical properties

Atterberg limits Liquid Limit Plastic Limit Plasticity Limit

26.9 %14.6%21.3 %

Linear Shrinkage 3.23Specific Gravity 2.61Compaction Optimum moisture content Maximum dry density

13.5 %1.85 Mg/m3

Type Gravel (2 to 60 mm) Sand (0.06 to 2 mm) Silt (0.002 to 0.06) Clay (<0.002 mm)

Size distribution10.0 %79.5 %7.5 %3.0 %

TABLE 2. General descriptions on species studied

SpeciesAcacia mangium Leucaena leucocephala

Depth of rooting (cm) 26.09 ± 4.54 37.02 ± 3.57Height of plant (cm) 265.09 ± 53.51 273.22 ± 36.21Maximum lateral root distance (cm) 45.02 ± 16.32 54.97 ± 17.73Total root length (cm) 460.53 ± 91.1 1256.62 ± 382.19Root length density (RLD) (cm m-3) 8.33 ± 1.52 13.65 ± 2.78

FiGurE 1. Root gripping by wedge and barrel

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Prior to the test, the dial gauge for the load ring and the vertical displacement were set to zero. Then, the root pull out was started by applying the pull out force at a constant rate (Figure 2). When the test are in progress, both dial gauges were observed and readings for the load ring were recorded on a Pull out Test form at every increment of 50 divisions’ of vertical displacement until the root system failed. The root pull-out resistance of the species studied and the relationship between pull-out resistance and various root morphologies were analyzed.

model (1972). All root samples of the selected plants were obtained manually by uprooting those plants in the field. The root diameter was measured by using vernier caliper and the fresh weight of the root sample was obtained by using an electronic balance. The Root Length Density (RLD) is calculated as total root length/soil volume. This parameter reflects the intensity of the root system.

RESULTS AND DISCUSSION

PULL-OUT RESISTANCE OF AcAcIA MAngIuM AND LeucAenA LeucocePHALA

In Acacia mangium, the maximum stem diameter of nine tested samples ranged from 17 to 33 mm, with maximum lateral spread of 49.5 mm from the base of the trunk and maximum depth of 25.56 cm, were chosen. The average of the pull-out resistance force increased drastically at the early stage of the test i.e. at small displacement, less than 20.0 mm (Figure 3). The lateral root of the plants were activated and provided most of the resistance for the plant. Subsequently, the gradient decreased gradually and the pullout resistance began to drop after reaching the maximum value, at about 1.5 kN. At the final stage, the irregular sounds of the root snapping were heard just before the plant was uprooted from the soil. In Leucaena leucocephala, the stem diameters of nine tested samples were used for the test, ranged from 18 mm to 35 mm. The lateral roots spread not more than 58.5 cm from the primary root and the depth was up to 27 mm. The average pull-out resistance was 2.25 ± 0.45 kN. Two distinct peak values were observed in the average of pull-out resistance force as against the displacement. The first peak (P1) value is due the maximum lateral root resistance whereby the second peak (P2) is the maximum mobilized tap root resistance (Figure 4). This is, arguably, one of the special features provided by the VH-type plant (Table 3) which generated by both lateral and tap roots. In general, the second peak value which is presumably due to the tap root seems to be higher than the first peak value. This observation indicates that the tap root plays a major role in providing the maximum pull-out resistance to this type of plant. Overall results imply that A. mangium only has single peak value. This species acquire the maximum strength of pull-out resistance from mainly the lateral roots. On the other hand, the pull-out resistance-displacement curve of L. leucocephala was slightly different whereby the two distinct peak values become visible, indicating the higher pull-out resistance can vary depending on the conditions and development of both lateral and tap roots. Between the two species, L. leucocephala exhibit the highest mean pull-out force (Table 4). Understanding the pull-out resistance of a plant is useful in our assessment of the ability of a plant to sustain environmental stress and forces such as wind, landslide, mass movement and soil creeping. The strength of plant roots especially the pull-out strength is important

FiGurE 2. Plant was pull-out from the soil

Root Tensile Test in the laboratory, a total of 36 and 21 root segments of A. mangium and L. leucocephala root samples were washed and then cut into 200 mm in length and the root was clamped with sand paper each time during the testing. These segments were grouped into ranges of 0.0-2.5 mm, 2.5-5.0 mm and 5.0-8.0 mm. The laboratory root tensile test was conducted by using the Universal Testing Machine (Instron, Model 5582, United Kingdom) to determine the root tensile strength. The roots were pulled up vertically at 500 mm/min in the testing equipment. During the test, measurement of force and extension at failure were measured. The value of tensile strength was derived as maximum force/sectional area of the root (N/mm2). Different parts of the root along its length, which correspond to different root sizes were sampled and tested. The maximum tension failure and the relationship between root tensile strength properties and root morphologies were also deduced.

ROOT ARCHITECTURE ANALySIS AND PROFILES

The root architecture (pattern) was determined through observation and comparison of the root morphologies other than branching patterns by using Yen’s description

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Displacement (mm)

Pull-

out r

esis

tanc

e fo

rce

(kN

)

FiGurE 3. Average pull-out resistance force-displacement curve for a pull-out test on five replicates A. mangium

Displacement (mm)

Pull-

out r

esis

tanc

e fo

rce

(kN

)

FiGurE 4. Average pull-out resistance force- displacement curve for a pull-out test on five replicates L. leucocephala. P1 and P2 contributed by lateral and tap roots, respectively

TABLE 3. Root growth pattern in trees (after yen 1972)

Species Illustration Root growth typeL. leucocephala VH-type

A. mangium H-type

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in natural condition i.e to withstand torrential runoff and prevent erosion as exhibited by Vetiveria zizanioides (Mickovski et al. 2007). It has also been discovered that most of the reinforcement in Vetiveria zizanioides is provided by the upslope lateral root that resists the force and prevent them from uprooting. A study carried out upon pull-out test with different model of root system by Stokes et al. (1996) discovered that the branching angle of the lateral root system lead to differences of uprooting force. This might explain the trend that has been discovered in this study, different composition of root distribution and branching between A. mangium and L. leucocephala can be as factors causing the differences in pull-out values. High force of pull-out exhibited by L. leucocephala shows the capability of the species to resist the uprooting forces. In tandem with L. leucocephala’s ability to establish on slope condition and reinforced the soil (Normaniza et al. 2008), the features to resist the uprooting helps the species anchorage and stability on slope against forces such as wind.

RELATIONSHIP BETWEEN PULL-OUT RESISTANCE AND PlANT MorPHoloGiES

All plant species were combined to determine the regression analysis. A linear relationship (R2 = 0.36; Figure 5) is observed between pullout capacity and stem diameter i.e. the pullout capacity increases as the diameter increasing. In addition, the existence of a linear (R2 = 0.36; Figure 6) relationship between pullout and

TABLE 4. Pull-out test and tensile strength of Acacia mangium and Leucaena leucocephala (data are means ± standard error)

Species Pull-out Force (kN) Tensile strength (N/mm2)Acacia mangium 1.69 ± 0.34 54.37 ± 10.80Leucaena leucocephala 2.25 ± 0.45 104.83 ± 18.72

Significant difference between species at lSDp<0.05 = 0.73 (pull-out) and 18.9 (tensile strength)

shoot dry weight also indicates that the maturity and the strength of the roots is depending on the development of the shoot. It is also hypothesized that Root Length Density (RLD) has a positive effect on the pull-out capacity. It is evidently shown in a strong linear relationship between pullout and root length density (R2 = 0.60; Figure 7) observed in this study. Overall regression analysis showed that the pull-out resistances of all species have a positive, either weak or strong, linear relationships with all the morphological properties of the plants. Bigger plants can resist pull-out force better than the smaller plants, which is to be expected given the relatively a constant root to shoot ratio. Plants that invest more in their above ground parts would also invest more in the proliferation of their root systems. The increase in pull-out resistance of plants that have root systems with extensive number of lateral root is due to the fact that the stronger soil-anchorage is developed by the lateral roots.

TENSilE STrENGTH

The root segments tested exhibited variation in value of tensile strength. Result of the tensile force of roots in each ranges of diameter shows that L. leucocephala exhibited the highest tensile force (Figure 8), with the average of 104.3 ± 18.72 N/mm2. Hence, based on the observation of tensile force at failure versus diameter, an increase in diameter increases the force (Figure 9). Similar trend was also observed by Tosi (2007). Final results showed that the average tensile strength (Force/root area) of L.

Stem diameter (mm)

Pull-

out r

esis

tanc

e (k

N)

FiGurE 5. Maximum pull-out resistance versus stem diameter

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Plant Shoot dry weight (g)

Pull-

out r

esis

tanc

e (k

N)

FiGurE 6. The relationship between the maximum pull-out resistance and the plant shoot dry weight

Root length density (cm m-3)

Pull-

out R

esis

tanc

e, F

(kN

)

FiGurE 7. Relationship between pullout resistance and root length density

Root diameter

Tens

ile F

orce

(N)

FiGurE 8. Tensile force at failure of roots according to different diameter classes

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leucocephala was the highest, almost double than that of A. mangium (Table 4).

rElATioNSHiP BETwEEN TENSilE STrENGTH ProPErTiES AND rooT MorPHoloGiES

From the regression analysis between tensile strength and root diameter, it can be observed that the tensile strength decreased with increasing root diameter (Figure 10) which is in line with the previous root studies (Baets et al. 2008; Genet et al. 2008; Tosi, 2007). During soil shearing, the finer and thinner roots that have high tensile strength would slip while the thicker roots with lower tensile strength would break (Mickovski & Van Beek 2009). It is reported by Genet et al. (2005) that this high resistance of the finer roots towards breakage was contributed by the higher cellulose content as been discovered in Pinus pinaster and castaena sativa. Thus, this property of higher tensile strength in the finer roots would ultimately yield an increase in shear strength of the root-soil composite in the field or natural condition. Hence, the finer roots will be able to provide better ductility to the root-soil composite with a higher capacity to withstand surface erosion and runoff (Mickovski & Van Beek 2009).

RELATIONSHIP BETWEEN PULLOUT AND TENSILE STrENGTHS

Most research carried out previously did not incorporate the pull-out and tensile strength. They only focused on either one of the test (e.g. Baets et al. 2008; Norris 2007; Tosi, 2007). Nilaweera & Nutalaya (1999) did both pull-out and tensile test, however they did not analyze the relationship between these two parameters. Interestingly, the results in this study showed that the pullout force is much affected by the tensile strength (R2 = 0.77; Figure 11).

rooT GrowTH PATTErN

The root growth pattern of the species studied had been determined during the studies (Table 2). The results provide important information as how tree anchorage is possibly affected by architecture. We discovered that the root branching pattern of A. mangium follows the model of root growth pattern by yen (1972) as it roots grow horizontally (H-type) and it may have three roles in slope stabilization: soil reinforcement, slope stability and wind resistance. In assessing the root architecture of L. leucocephala, its root system seem more prominent in terms of depth of

Root diameter (mm)

Tens

ile fo

rce

(N)

FiGurE 9. Tensile force at failure versus root diameter

Root diameter (mm)

Tens

ile st

reng

th (N

/mm

2 )

FiGurE 10. Tensile strength versus root diameter

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rooting, total root length, maximum lateral root distance and root length density (Table 2) which establish vertically and horizontally (VH-type), respectively, as compared to A. mangium. In comparison, root architecture of L. leucocephala is more prominent in the deeper soil depth as it has a combined reinforcement effect of both long tap and extensive lateral roots.

CONCLUSION

The pull-out resistance is observed to be affected by both shoot and root profiles. However the parameter studied was much affected by the root (Root Length Density) rather than the shoot profiles (e.g. shoot dry weight and stem diameter), indicating that root profile gives more attribution to pull-out capacities as compared to shoot. The results also imply that the soil stability will increase with time as the root system grows. As for the tensile strength, the value decreases with increasing root diameter. The results also showed that the pullout force is much affected by the tensile strength. It can be concluded that some root and shoot properties do have a great impact on root mechanical properties such as tensile and pullout strengths. In both parameters, L. leucocephala exhibits the highest values. The study also suggests that L. leucocephala has an added value as a good potential slope plant for slope stabilization work i.e. high resistance towards torrential runoff and prevent erosion, as it exhibits outstanding root mechanical properties.

ACKNowlEDGEMENTS

This research was funded by Slope Engineering Branch, Public Works Department, Malaysia and partly sponsored by Ministry of Science, Technology and Innovation (the 9th Malaysian Plan).

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Tensile strength (N/mm2)

Pull-

out F

orce

(kN

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FiGurE 11. Relationship between pull-out forces versus root tensile strength

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Normaniza Osman* & Mohamad Nordin AbdullahInstitut Sains BiologiFakulti SainsUniversiti Malaya50603 Kuala lumpurMalaysia

Che Hassandi AbdullahKetua Unit Penyelidikan dan PerkembanganCawangan Kejuruteraan CerunTingkat 12, Blok F, Ibu Pejabat JKRJalan Sultan Salahuddin 50582 Kuala LumpurMalaysia

*Corresponding author; email: normaniza@um.edu.my

Received: 21 May 2010Accepted: 3 November 2010